One of the technical problems which the invention attempts to solve is the compromise between the viscosity level which polymers exhibit in the molten state and their mechanical properties. This is because, in the thermoplastics technology, for example, a low viscosity is required during the processing, which can only be obtained with systems of low molecular weight, but, at the same time, a high elongation at break, a good impact strength, a good creep strength and good resistance to solvents are desired for the final material, which, conversely, requires high molecular weights.
Moreover, it is well known that the synthesis of polymers of very high molecular weights is markedly more difficult than that of polymers of low weights.
Another technical problem to be solved is the preparation of polymer compositions having an improved chemical resistance and an improved chemical stability.
Another problem which the invention attempts to solve is the preparation of block or comb sequential copolymers by joining up homopolymers prepared in a first step.
The Applicant company has found that these problems can be solved by the chemical modification of polymers by introducing therein a modifying agent comprising imidazolidone groups.
The chemical modification of polymer systems according to the invention involves the formation of a covalent bond between the polymer backbone and the modifying agent. The modifying agent comprises, combined in the same molecule, one or more associative groups capable of uniting via hydrogen bonds and one or more reactive groups capable of forming covalent bonds with the polymer backbone. According to the invention, at least one of the associative groups of the modifying agent is a heterocycle of imidazolidone type. If appropriate, the polymer backbone under consideration can comprise a certain number of comonomers carefully chosen in order to facilitate the grafting operation, that is to say the attaching of the modifying agent to the polymer backbone via at least one covalent bond.
Due to the presence of the associative groups, the modified polymer chains are capable of uniting via cooperative hydrogen bonds; the role of the associative groups may also be to strengthen this associative ability if the latter preexists in the unmodified system. One advantage of this type of association is that these hydrogen bonds are reversible: they can be cleaved by a rise in temperature or by the action of a selective solvent and can subsequently be reformed.
One advantage of the chemical modifications disclosed in this patent is the improvement in the conditions for processing and the final properties, in particular mechanical properties, of the modified polymers. Likewise, the use of associative groups confers the properties of polymers of high weights on polymers of small molecular weights which are easier to produce in a controlled fashion.
The chemical modifications disclosed in this patent make it possible to graft the same type of associative group to different types of macromolecular chains.
The associative groups can be introduced into the polymers by different routes. Mention may be made of copolymerization with a monomer carrying the associative group or the use, during the synthesis of the polymer, of the modifying agent as transfer agent or terminating agent. According to the invention, the associative groups are introduced onto preformed polymer or copolymer systems by reaction with the modifying agent; in other words, after the polymerization or polycondensation stage. For the polymer systems resulting from polycondensations, such as, for example, polyesters and polyamides, which de facto exhibit reactive chain ends, the modifying agent can also be introduced during the synthesis as chain-limiting agent.
One of the advantages of proceeding by modification of existing polymers or copolymers is to be able to synthesize an entire range of products from a single polymerization; for example, modified polymers comprising a relatively large number of associative groups can be obtained by varying the amount of modifying agent or the reaction time during the grafting stage, whereas, to achieve the same result using monomers carrying the associative group, it is necessary to carry out a fresh polymerization in each case.
Finally, the presence of these associative groups on macromolecular chains belonging to different polymer categories makes it possible to produce block copolymers, the two blocks of which are linked via hydrogen bonds.